Liquid crystal display is occupying an important position, taking the place of the cathode-ray tube, because of such characteristic features as being driven at a low voltage, light weight and low cost. However, since the liquid crystal display adopts a means of displaying images by utilizing orientating behaviors of a liquid crystalline substance having an optical anisotropy, it inevitably involves the problem (hereinafter referred to as "viewing angle dependency") that the color tone of the screen changes according to direction of viewing the panel. That is, the visual angle dependency becomes more conspicuous when a color compensating optical elements such as a stretched film or a liquid crystal cell is mounted to the liquid crystal cell. For example, in the case of a lap-top type personal computer or a word processor display which utilize a twisted nematic mode ("TN" hereinafter) or a super-twisted nematic mode ("STN" hereinafter) liquid crystal cell, relatively good images are obtained when the screen is seen from the front. However, when the screen is seen from a direction other than the front, the screen is colored or it becomes difficult to see a clear image on the screen. Such viewing angle dependency of the display is not only undesirable to users but also is an obstacle to the production of a larger screen which is required for a wall-mounted TV or the like. In the case of a large screen display, as long as the prior art is used, it is impossible to obtain clear images throughout the entire screen because the marginal portion comes to have a certain viewing angle dependency even when the screen is seen from the front.
As a mean for diminishing such visual angle dependency of the display, it has been suggested to use a film having a refractive index in the thickness direction which larger than an intra-plane refractive index [M. Akatuka et al.: Japan Display '89, 336 (1989)]. Actually, however, such a film is scarcely existent, and even when it does exist, the difference between the refractive index in the thickness direction and the intra-plane refractive index is 0.001 or so (Japanese Patent Laid Open No. 85519/1991). Hence no effective remedy for the viewing angle dependency of liquid crystal displays have been yet obtained.
As a substance having a refractive index anisotropy, a liquid crystalline substance is promising however certain problems arise using such liquid crystalline substances. For example, in the case of a low molecular liquid crystal, it is difficult to maintain a stable orientation whereas in the case of a high molecular liquid crystal, it is impossible to obtain a uniform orientation.
The present inventors have already proposed an optical element constituted by a nematic liquid crystalline polymer film having a twisted structure as a color compensator for TN for liquid crystal display (Japanese Patent Application No. 165718/1990) and STN liquid crystal display (Japanese Patent Laid Open No. 87720/1991). But for further development of a viewing angle compensator for liquid crystal display having a higher function, the present inventors have taken note of a liquid crystalline polymer film having a large refractive index in the thickness direction and made extensive studies thereon. As a result, we accomplished the present invention.
It is the object of the present invention to provide a viewing angle compensator for a liquid crystal display unit and more particularly to provide a viewing angle compensator for a liquid crystal display unit using a liquid crystalline polymer film having a structure wherein a refractive index in the thickness direction is larger than a refractive index at least in one direction within the plane of the polymer and having an orientation state that is solidified.
Another object of the present invention is to provide a process for compensating the viewing angle display using the aforementioned liquid crystalline polymer film.